1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus employing the optical scanning device, such as a digital copying machine, a laser printer, and a laser facsimile.
2. Description of the Related Art
Generally speaking, in optical scanning devices that are popularly used in image forming apparatuses such as laser printers to which an electronic photograph process is applied, a light beam emitted from the light source side is deflected by an optical deflector, and the deflected light beam is converged onto a scanning surface by a scanning and imaging forming optical system like an Fθ lens, so that a beam spot is formed on the scanning surface, and the scanning surface is optically scanned by the beam spot (this scanning process is called “main scanning”). The actual substance of the scanning surface is a photosensitive surface of a photosensitive medium that includes a photosensitive member having photoconductivity.
As an example of full-color image forming apparatuses, a “tandem-type image forming apparatus” is publicly known in which four photosensitive members are arranged along the conveyance direction of a recording paper, and an image is formed by deflecting and scanning, using a single deflecting device, a light flux of light beams that are emitted from a plurality of light source devices and correspond to these photosensitive members. In such a “tandem-type image forming apparatus”, latent images are formed through a simultaneous exposure process with image signals for color components corresponding to the photosensitive members, using a plurality of scanning and image forming optical systems respectively corresponding to the photosensitive members. The latent images are then made into visible images by developing devices that use developers in mutually different colors, namely in yellow, magenta, cyan, and black, for example. Subsequently, the visible images are sequentially transferred and fixed onto a same sheet of recording paper on top of one another, so that a color image is obtained. Of tandem-type image forming apparatuses in which two or more sets of an optical scanning device and a photosensitive member are used in combination so as to form two-color images, multi-color images, and color images, in some tandem-type image forming apparatuses that are publicly known, a single optical deflector is used in common among a plurality of photosensitive media, as described below.                (1) A plurality of light fluxes that are substantially parallel to one another and are apart from one another in a sub-scanning direction are made incident to an optical deflector. A plurality of scanning optical devices that correspond to the light fluxes are arranged in the sub-scanning direction so that a scanning process is performed (see, for example, Japanese Patent Application Laid-Open No. H9-54263).        (2) Light fluxes are made incident to an optical deflector from one side of the optical deflector. A scanning optical system includes three lenses. A plurality of light fluxes that travel toward mutually different scanning surfaces pass through a first lens L1 and a second lens L2. A third lens L3 is provided for each of the light fluxes that travel toward the mutually different scan surfaces (see, for example, Japanese Patent Application Laid-Open No. 2001-4948, Japanese Patent Application Laid-Open No. 2001-10107, and Japanese Patent Application Laid-Open No. 2001-33720).        
By using a single optical deflector in common for a plurality of scanning surfaces, it is possible to reduce the number of optical deflectors and to make the image forming apparatus compact. However, for optical scanning devices to be included in a image forming apparatus for full-color images having scanning surfaces (photosensitive members) that respectively correspond to four colors, namely for example, cyan, magenta, yellow, and black, even if it is possible to reduce the number of optical deflectors, a problem still remains where the size of the optical deflector, namely, for example, a polygon mirror, needs to be large in the sub-scanning direction because of the arrangement in which the light beams traveling toward the photo sensitive members are made incident to the optical deflector while they are arranged in a row in the sub-scanning direction, being substantially parallel to one another. Generally speaking, the cost of the polygon mirror portion among optical elements included in an optical scanning device is high. In an endeavor to reduce the costs and the size of an optical scanning device as a whole, a large polygon mirror creates a problem.
Recently, one of the systems that are publicly known for reducing the costs by using a single optical deflector in an optical scanning device included in a color image forming apparatus is an oblique incident optical system with which a light beam is made incident to the deflecting reflection surface of the optical deflector at an angle in the sub-scanning direction (see, for example, Japanese Patent Application Laid-Open No. 2003-5114). In the oblique incident optical system, after being deflected and reflected on the deflecting reflection surface, each of a plurality of light beams that have been separated by a reflection mirror or the like is introduced to a corresponding one of scanning surfaces (photosensitive members). When the light beams are separated, the angle, in the secondary direction, of each of the light beams (i.e. the angle at which each of the light beams is obliquely incident to the optical deflector) is set at such an angle that the light fluxes can be separated by the reflection mirror. When this type of oblique incident optical system is used, it is possible to avoid using a large optical deflector, in other words, it is possible to avoid using a polygon mirror having too many layers or being too thick in the sub-scanning direction, while having an arrangement in which the light fluxes can be separated by the mirror, and also the distance between the light beams that are positioned adjacent to one another in the sub-scanning direction is kept small.
When a polygon mirror is used as an optical deflector, it is difficult, with a normal incident method, to make the light fluxes emitted from the light source side incident toward the rotation axis of the polygon mirror. It is not impossible to make the light fluxes incident toward the rotation axis of the polygon mirror; however, assuring a sufficient deflection angle requires that each of the deflecting reflection surfaces becomes extremely large and makes it impossible to keep the size of the polygon mirror small. When the size of the polygon mirror is large, the degree of occurrence of what is called “sags” becomes large, too. The sags occur asymmetrically for an image height 0. When the size of the polygon mirror is large, a lot of energy is required for a high-speed rotation of the polygon mirror, and a noise preventing means needs to be large, too, because the “whistling noise” during the high-speed rotation is also loud.
To the contrary, when an oblique incident method is used, because it is possible to make the light flux from the light source side incident toward the rotation axis of the polygon mirror, it is possible to make the diameter of the polygon mirror small. The “whistling noise” during a high-speed rotation is also small. Accordingly, the oblique incident method is suitable for achieving a high speed. Because it is possible to make the diameter of the polygon mirror small, the degree of occurrence of sags is small, too. Also, because it is possible to make sags occur symmetrically for an image height: 0, it is also easy to correct the sags.
However, the oblique incident method has the problem of having a large “bending of a scanning line”. The amount of occurrence of the bending of scanning line varies depending on the oblique incident angle, in the sub-scanning direction, of each of the light beams. When latent images that have been drawn by the light beams are made visible with toners for different colors and are overlapped on top of one another, the images will exhibit a color registration error. Also, when the oblique incident method is used, because the light flux is incident while being distorted with respect to a scanning lens, the wave aberration increases, and the level of optical performance is significantly degraded especially for the periphery image height. Thus, the beam spot diameter for the periphery image height becomes large, and it could be one of the causes that hinder the endeavor to make high quality images.
Some methods have been proposed to correct a “large bending of a scanning line”, which is a problem unique to the oblique incident method. As an example, “a lens that has a lens surface of which the unique inclination in the sub-scanning cross-sectioned plane is altered toward the main-scanning direction so that the bending of the scanning line is corrected” is added to a scanning and image forming optical system (see, for example, Japanese Patent Application Laid-Open No. H11-14932). As another example, “a correcting reflection surface that has a reflection surface of which the unique inclination in the sub-scanning cross-sectioned plane is altered toward the main-scanning direction so that the bending of the scanning line is corrected” is added to a scanning and image forming optical system (see, for example, Japanese Patent Application Laid-Open No. H11-38348).
Another method that has been proposed is to let a light flux being obliquely incident to an optical deflector pass on the outside of the axis of a scanning lens, and to bring the positions of the scanning lines in alignment by using a surface by which the amount of asphericity of the non-generatrix (radius curvature of a sub-scanning) of the scanning lens changes along the main-scanning direction (see, for example, Japanese Patent Application Laid-Open No. 2004-70109). This publication discloses an example in which a correction process is performed by one scanning lens. With this arrangement, it is possible to correct the bending of the scanning line; however, the publication does not mention degradation of a beam spot diameter due to an increase in the wave aberration.
Another problem related to the oblique incident method is that the wave aberration is easily degraded by a large amount for the periphery image height (near both ends of a scanning line) due to a light beam skew. When a wave aberration occurs, the spot diameter of a beam spot for the periphery image height becomes large. Unless this problem is solved, it is not possible to achieve “a high-density optical scanning”, which is strongly demand these days. The optical scanning device disclosed in the Japanese Unexamined Patent Application Publication No. 2004-70109 is able to correct extremely well a large bending of scanning line, which is a problem unique to the oblique incident method, but is not able to correct the wave aberration in a sufficient manner.
Another optical scanning device has been proposed to properly correct “the bending of the scanning line and degradation of the wave aberration”, which are the problems related to the oblique incident method. In this proposed apparatus, the scanning and image forming optical system includes a plurality of rotating asymmetric lenses, and the shape of a generatrix that connects the vertices of the non-generatrix on the lens surface in the sub-scanning direction, is configured to be curved in the sub-scanning direction (see, for example, Japanese Patent Application Laid-Open No. H10-73778).
However, as for the lens having the “lens surface on which the shape of the generatrix is configured to be curved in the sub-scanning direction”, the problems are solved by having the generatrix curved. Thus, it is necessary to have individual scanning lenses each of which corresponds to a different one of incident light fluxes. Consequently, when the optical scanning device is used as a tandem scanning optical system, the number of scanning lenses to be used becomes large.
When a plurality of light fluxes that travel toward mutually different scanning surfaces are made incident to a single lens, with the arrangement in which the shape of the generatrix is curved, it is possible to solve various problems for a light flux on one side, but it is difficult to reduce the bendings of the scanning lines and the wave aberration for a light flux on the other side.
Also, because the lens has a bending in the sub-scanning direction, when the light flux being incident to the lens is shifted in the sub-scanning direction, the shape of the bending of the scanning line is changed because of a refracting power of the lens in the sub-scanning direction, due to the influence of assembly errors, process errors, environment variation, or the like. Thus, it is not possible to achieve the effect of inhibiting color registration errors in color images, the effects being expected at the initial stage (or at the designing stage), and the problem of a color registration error arises.
Further, in the process of correcting the wave aberration, on a surface having a bending, the degree of the light flux skew largely varies due to instability of the incident light flux. Consequently, it is difficult to achieve a good beam spot diameter constantly.
According to the invention disclosed in Japanese Patent Application Laid-Open No. 2003-5114 that uses the oblique incident method, a bending of a scanning line is corrected using the same type of surface according to the invention disclosed in Japanese Patent Application Laid-Open No. H10-73778; however, as described above, it is difficult to achieve a good beam spot diameter constantly with this arrangement.
In addition, when an oblique incident optical system is used, a bending of a scanning line is observed when there is a change in the temperature. Because a light beam is incident to a scanning lens while being curved in the sub-scanning direction, the change in the temperature causes a change in the bending radius of the scanning lens, a change in the thickness of the scanning lens, or a change in the incident height of the light beam being incident to the scanning lens; therefore, the degree of the bending in the scanning is large. When a scanning optical system according to a conventional technology is used, a light beam is incident to a scanning lens substantially horizontally with respect to the optical axis without being curved, there is no bending of the scanning line, or if any, the degree of the bending of the scanning line is extremely small. Accordingly, the problem of bendings of scanning lines is a problem unique to oblique incident optical systems.
The inventions disclosed in the above literatures aim to make the degree of bendings of scanning lines small but do not solve the problem of bendings of scanning lines caused by a change in the temperature.